Mosaic mutations of RUNX2 have not been previously described in CCD patients, but because ~32% of DNA molecules in our patient's blood harbor the deletion, our finding is consistent with the report of a mouse model where ≥30% reduction in Runx2 wild‐type messenger RNA (mRNA) level produces a CCD phenotype.(28) Our results suggest that the level of RUNX2 mosaicism in patient 2's blood cells is 64%. This evidence concerns the gene RUNX2 and cleidocranial dysplasia 1.